Estimating Egg Retention Times in Sceloporine Lizards

Abstract

The extent to which squamates retain eggs in utero is poorly known. The purpose of this study was to develop a broadly applicable laboratory based method for estimating egg retention time relative to the total period of embryonic development. The first step in determining the range of egg retention times for a species is quantifying embryo stage at normal oviposition for a large sample of females. The next step involves measuring the total time taken to develop, from ovulation to hatching, at constant temperature. Finally, by calculating the fraction of total development time taken to achieve each embryonic stage from ovulation, relative egg retention time can be determined. The modal embryo stages at oviposition in Sceloporus woodi, S. virgatus, and S. scalaris are stage 27, 31, and 38, respectively. It was previously reported that total embryonic development times at 30 C for these three species are 61.6, 55.3, and 45.6 days, respectively. Additionally, embryos of each species achieve similar stages at similar pro- portions of total development time. Embryo stages at oviposition (ESO), reported in the literature, indicate that the modal ESO for lizards (N = 38 species) is stage 30, which corresponds to approximately 26% of total embryonic development time. Although there are a few species of squamates that exhibit extreme periods of egg retention, most squamates appear to retain eggs for an absolute period that may be slightly longer than the egg retention times of crocodilians and turtles. Unlike turtles and crocodilians, squamate embryos continue to develop in utero and achieve a more advanced degree of embryogenesis at oviposition